The invention relates to a facet mirror device that may be used within an optical device used in exposure processes, in particular in microlithography systems. It further relates to an optical imaging arrangement comprising such a facet mirror device. It further relates to a method of supporting a facet element of a facet mirror device. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.
Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical element modules comprising optical elements, such as lenses, mirrors, gratings etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to illuminate a pattern formed on a mask, reticle or the like and to transfer an image of this pattern onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups that may be held within distinct optical element units. Facet mirror devices as the ones mentioned above, among others may serve to homogenize the exposure light beam, i.e. to effect a power distribution within the exposure light beam which is as uniform as possible. They may also be used to provide any desired specific power distribution within the exposure light beam.
Due to the ongoing miniaturization of semiconductor devices there is not only a permanent need for enhanced resolution but also a need for enhanced accuracy of the optical systems used for fabricating those semiconductor devices. This accuracy obviously not only has to be present initially but has to be maintained over the entire operation of the optical system. A particular problem in this context is proper heat removal from the optical components to avoid uneven thermal expansion of these components leading to uneven deformation of these components and, ultimately, to undesired imaging errors
As a consequence highly sophisticated facet mirror devices have been developed such as they are disclosed, for example, in DE 102 05 425 A1 (Holderer et al.), the entire disclosure of which is incorporated herein by reference.
DE 102 05 425 A1 (Holderer et al.), among others, shows facet mirror devices where facet elements with a spherical rear surface sit in an associated recess within a support element. The spherical rear surface rests against a corresponding spherical wall or a plurality of contact points of the support element confining this recess. The spherical rear surface has a comparatively small radius of curvature such that it defines a center of rotation of the facet element that is located far remote from a center of curvature of the optical surface of the facet element.
A manipulating lever is centrally connected to the rear surface of the facet element and corresponding manipulators tilt the manipulating lever, i.e. generate lateral excursions of the free end of the manipulating lever, to adjust both the position and the orientation of the optical surface of the facet element.
This configuration, however, has the disadvantage that an adjustment of the orientation of the optical surface of the facet element by tilting the manipulating lever typically leads to a frictional relative motion between the facet element and the support element. This frictional relative motion leads to the introduction of undesired parasitic forces and moments, respectively, into the facet element.
Furthermore, in some cases, the manipulating lever is used as a tensioning element for fixing the facet element relative to the support element once it has been adjusted through a clamping force pressing the facet element against the support element. To this end, a spring device contacting the support element acts on the free end of the manipulating lever to put the latter under tension along its longitudinal axis thereby pressing the facet element against the support element.
Such a clamping connection, compared to an adhesive connection, has the advantage that it may be easily achieved and also released for re-adjustment, and that it is free from undesired effects such as glue shrinkage (leading to an introduction of parasitic stresses into the facet element) and outgassing of contaminants.
The clamping configuration disclosed in DE 102 05 425 A1 (Holderer et al.), however, has the disadvantage that, depending on the degree of rotation about the center of rotation defined by the spherical rear surface, either one of the end of the spring device contacting the manipulating lever and the manipulating lever is laterally deflected. Hence, the tensioning force generated by the spring device has a component in a direction transverse to the longitudinal axis of the manipulating lever. As a consequence, the laterally deflected spring device and the bent manipulating lever generate a resetting moment about the center of rotation counteracting the adjustment motion. This results in undesired parasitic moments introduced into the facet element via the manipulating lever.